Suspension is an essential factor which affects the driving stability of a vehicle, and its main components includes a link (an arm and a rod) configured to vertically movably support the wheels for a vehicle body, a spring configured to absorb an impact from a road surface by bending, and a damper configured to damp the oscillation of the spring. A damper for vehicle suspension includes a cylindrical-shaped cylinder filled with hydraulic fluid, a piston configured to slide in the cylinder in its axis direction, and a piston rod having an end to which the piston is attached. A typical damper has a double or single cylinder structure in which hydraulic fluid is moved between a plurality of fluid chambers along with the operation of the piston.
In a cylindrical damper, a piston is provided with a disc valve which is flexible and has a communicating fluid passage. In general, damping force is obtained by applying a flow resistance by the disc valve to the hydraulic fluid which is moved between the fluid chambers via the communicating fluid passage. However, such a damper has fixed damping characteristics, and thus it is not possible to obtain ride comfort and driving stability corresponding to a road surface state and driving conditions. Thus, a variable damping-force damper has been proposed in which valve plates composed of a magnetic material on the contraction and extension sides are installed on the upper and lower surfaces of the piston body, a circular magnetic coil to generate a magnetic field is interposed between an outer yoke and an inner yoke which constitute the piston body, and the strength of the magnetic field changes with an increase or decrease of the amount of current through the magnetic coil, thereby continuously changing the valve opening characteristics (i.e., damping force) of the valve plates (see Japanese Patent No. 4599422 and Japanese Patent No. 4825723).